Method for the decarboxylation of dicarboxylic acids

ABSTRACT

The invention relates to a method for the thermal decarboxylation of dicarboxylic acids, in particular to 3,4-ethylene dioxythiophene-2,5-dicarboxylic acid as an educt. According to said method the educt is used in solid form and/or the reaction is carried out in the presence of a plurality of fluidised bed bodies. No solvents are used in the reaction and the decarboxylation product that is formed during the reaction is carried away from the reaction zone in gaseous form.

The invention relates to a process for the thermal decarboxylation ofdicarboxylic acids, in particular the decarboxylation ofdialkoxythiophenedicarboxylic acids andalkylenedioxythiophenedicarboxylic acids without addition of anadditional solvent, if appropriate with addition of an inert gas.

For the purposes of the present invention, inert gases are gases whichdo not undergo any reaction with the dicarboxylic acid under theconditions employed.

Various methods of decarboxylating monocarboxylic and polycarboxylicacids are known. According to U.S. Pat. No. 2,453,103, the thermaldecarboxylation of 3,4-dimethoxythiophene-2,5-dicarboxylic acid iscarried out with addition of a pulverulent copper catalyst. Similarprocedures for the decarboxylation of dialkoxythiophenedicarboxylicacids and alkylenedioxythiophenedicarboxylic acids are described, forexample, by Coffey et al. (thermal decarboxylation of the moltenmaterial, in: Synthetic Communications, 26(11), 2205-2212, 1996), byStéphan et al. (decarboxylation at 180° C. with addition ofchromium(III)-copper(II) oxide, in: Journal of ElectroanalyticalChemistry, 443, 217-226, 1998) or Merz and Rehm (without catalyst ordiluent, in: Journal für Praktische Chemie, 228, 672-674, 1996). Theconditions employed in these processes usually take account of thethermal instability of the compounds. Thus, the decarboxylation isadvantageously carried out in a solvent with addition of a catalystwhich accelerates the decarboxylation. The decarboxylation of3,4-ethylenedioxythiophene-2,5-dicarboxylic acid to form3,4-ethylenedioxythiophene is thus carried out in an organic solvent(for example tetrahydrothiophene 1,1-dioxide) with addition of a coppercarbonate as catalyst at temperatures of 100-200° C. and pressures of800-1200 hPa. In this way of carrying out the process, all of thestarting material is firstly decarboxylated and the product is thendistilled from the solvent under reduced pressure. However, in additionto the 3,4-ethylenedioxythiophene-2,5-dicarboxylic acid, the startingmaterial always contains small amounts of by-products which are formedin previous steps in the synthesis. These by-products accumulate in thesolvent and limit the reusability of solvent and catalyst. Cu-containingwaste is formed and has to be disposed of, which costs money.Furthermore, the process described for the decarboxylation of3,4-ethylenedioxythiophene-2,5-dicarboxylic acid, which represents theprior art, can be operated only batchwise. In the purely thermaldecarboxylation, a large part of the product remains in the reactionspace and, owing to the elevated temperatures, there is increasedformation of by-products which have a severe adverse effect on thequality or make complicated subsequent purification necessary.

It is an object of the present invention to develop a procedure for thedecarboxylation of 3,4-ethylenedioxythiophene-2,5-dicarboxylic acid andsimilar dicarboxylic acids which makes a continuous or pseudocontinuousmode of operation possible while simultaneously minimizing the use ofauxiliaries (solvent and catalyst).

The invention achieves this object by means of a process for the thermaldecarboxylation of dicarboxylic acids, in particular3,4-ethylenedioxythiophene-2,5-dicarboxylic acid, as starting material,characterized in that the starting material is used as a solid and/orthe reaction is carried out in the presence of a plurality offluidized-bed bodies, with the reaction being carried out in the absenceof solvents and the decarboxylation product formed in the reaction, inparticular 3,4-ethylenedioxythiophene, being discharged from thereaction zone in gaseous form.

The decarboxylation is preferably carried out at a temperature of from100 to 600° C., more preferably from 100 to 500° C., particularlypreferably from 150 to 400° C.

In experiments on the purely thermal decarboxylation of3,4-ethylene-dioxythiophene-2,5-dicarboxylic acid in a fluidized bed, ithas surprisingly been found that 3,4-ethylenedioxythiophene can beobtained in very high selectivities even in the absence of a solvent byheterogeneous reaction of the solid starting material alone. It is foundthat the 3,4-ethylenedioxythiophene formed has a sufficiently high vaporpressure at the temperature necessary for the decarboxylation to be ableto be discharged in gaseous form together with an inert gas stream andbe able to be precipitated by cooling. In this way, the use ofauxiliaries is minimized and continuous or pseudocontinuous operation ismade possible.

The process of the invention can be carried out in various types ofreactor, as long as the product formed by means of the decarboxylation,e.g. 3,4-ethylene-dioxythiophene, can be discharged from the reactor ingaseous form. Examples which may be mentioned here are fixed-bedreactors, moving-bed reactors, reactors containing a bubble-forming,turbulent or jet-permeated fluidized bed, internally or externallycirculating fluidized beds. It is also possible to introduce thestarting material, e.g. 3,4-ethylenedioxythiophene-2,5-dicarboxylicacid, into a reactor filled with fluidized bed bodies which, forexample, comes under the abovementioned classes.

In particular, the process is carried out continuously in abubble-forming or turbulent or jet-permeated fluidized bed, or in aninternally or externally circulating fluidized bed.

The reaction is particularly preferably carried out in the presence ofan inert auxiliary gas, in particular a gas selected from the groupconsisting of noble gases, nitrogen, water vapor, carbon monoxide andcarbon dioxide and mixtures of various such inert auxiliary gases.

Possible inert gases include all gases which do not react with thestarting material or product under the reaction conditions selected;suitable inert gases, for example, noble gases, nitrogen, water vapor,carbon monoxide or carbon dioxide. It is possible to carry out theprocess of the invention for the decarboxylation of3,4-ethylenedioxythiophene-2,5-dicarboxylic acid with addition of aninert gas or a mixture of a plurality of inert gases in any combination.

The temperature can, as described, be varied within the temperaturerange from 100° C. to 600° C. However, it has to be high enough for thedecarboxylation of the starting material, e.g.3,4-ethylenedioxythiophene-2,5-dicarboxylic acid, to be achieved andmust not exceed the decomposition temperature of3,4-ethylene-dioxythiophene-2,5-dicarboxylic acid or3,4-ethylenedioxythiophene.

The reaction is preferably carried out in a fluidized-bed reactor inwhich fluidized bed bodies having a mean diameter (number average)greater than the particle diameter of the dicarboxylic acid are used.

The fluidized bed bodies particularly preferably have a solids densityρ_(s) of 0.5 g·cm⁻⁵<ρ_(s)<6 g·cm⁻³.

The fluidized bed bodies can also preferably be used as heat transfermedia which are preheated outside the reaction zone and circulatedthrough the reaction zone.

The fluidized bed bodies preferably consist partly or entirely of acatalytically active material, in particular copper or a copper salt,preferably CuCO₃.

The process of the invention is preferably carried out in a fluidizedbed. For this purpose, solid particles of3,4-ethylenedioxythiophene-2,5-dicarboxylic acid, hereinafter referredto as particles, are placed in the reaction space. The particles can beintroduced batchwise or continuously from the outside. The particlesform a fixed bed through which the gas fed in is passed. The inflowvelocity of the gas fed in can be set so that the fixed bed is fluidizedand a fluidized bed is formed. The appropriate procedure is known per seto those skilled in the art. The inflow velocity of the gas fed in hasto correspond at least to the loosening velocity (also referred to asminimum fluidization velocity). For the present purposes the looseningvelocity is the velocity at which a gas flows through a bed of particlesand below which the fixed bed is retained, i.e. below which the bedparticles remain largely stationary. Above this velocity, fluidizationof the bed commences, i.e. the bed particles move and the first bubblesare formed. In operation of a bubble-forming fluidized bed, the gasvelocity is selected so that it corresponds to from one to ten times theloosening velocity, preferably from one to seven times the looseningvelocity, particularly preferably from one to five times the looseningvelocity.

If the starting material is present in a very fine particle fraction,then interparticulate forces predominate and the solids are difficult tohandle. In this case, it can be advantageous to introduce the3,4-ethylenedioxythiophene-2,5-dicarboxylic acid starting material intoa bed of relatively coarse particles. If the mean particle size of the3,4-ethylenedioxythiophene-2,5-dicarboxylic acid is, for example, 0.1-50μm, then the handling in a bed containing particles having a meanparticle diameter dP of 50<dP<350 μm can be made considerably easier.These relatively coarse bed particles preferably have a solids densityρs of 0.5 g·cm⁻³<ρs 4 g·cm⁻³. These described particles act as carriersfor the small particles which adhere to the surface. Furthermore, theinitially charged bed particles can serve as momentum carriers and heattransfer media and/or consist entirely or partly of a catalyticallyactive material. As catalytically active materials, it is possible touse, for example, metals, metal oxides or metal salts. Particularpreference is given to using copper, copper oxides and copper salts. Itis possible to use all-active materials, but the active components canalso be mixed with or applied to a support. The use of pure componentsor of mixtures is conceivable. The3,4-ethylenedioxythiophene-2,5-dicarboxylic acid starting material canbe introduced, for example, by means of screws, injectors or a locksystem. A further method of introduction is the spraying-in of molten3,4-ethylenedioxythiophene-2,5-dicarboxylic acid or3,4-ethylenedioxythiophene-2,5-dicarboxylic acid which has been dilutedwith a solvent.

The introduction of a catalyst in pure form, as a mixture or insupported form can also be effected in the form of internals in thereactor. Examples which may be mentioned are rigid or flexibleinternals, rod- and tube-shaped internals, constructions in the form ofperforated metal sheets, meshes, grids or three-dimensional structuresand also packing and the use of shaped bodies or free-flowing elements.

The separation of finely-divided solids starting material from the gasstream leaving the reactor can be effected, for example, by means of acyclone, a filter or a gas scrubber. It is preferably separated off bymeans of a cyclone and/or a filter. The collected solids startingmaterial is advantageously recirculated to the decarboxylation step.This recirculation can be effected, for example, by means of internal orexternal circulation. However, recirculation can also be carried out byback-blowing and cleaning of filters.

A preferred process is characterized in that any solid carried out fromthe reaction zone by the gas stream is separated off from the product bymeans of a cyclone and/or filter.

Preference is likewise given to a variant of the process in which theunreacted solid starting material which has been circulated off from theproduct gas stream is recirculated either batchwise or continuously tothe reaction zone.

The process of the invention is illustrated below with the aid of someexamples, but the examples are not to be regarded as restricting thescope of the invention.

EXAMPLES Example 1 Fixed Bed

4 g of dried crude 3,4-ethylenedioxythiophene-2,5-dicarboxylic acid wereplaced in a glass reactor (diameter: 16 mm, total height: 400 mm) andheated to a maximum reaction temperature of 300° C. (heating rate: 2°C./min) in a stream of nitrogen. The starting material was reacted overa time of 80 minutes. Of the 4 g (1.7×10⁻² mol) initially introduced,3.5 g (1.52×10⁻² mol) had reacted and a small proportion was carried outfrom the reactor by means of the nitrogen stream. 2.89 g of a productmixture of 3,4-ethylenedioxythiophene-2,5-dicarboxylic acid and3,4-ethylenedioxythiophene were collected in a downstream cold trap. The3,4-ethylenedioxythiophene content determined by HPLC was >90% byweight.

Example 2 Fluidized Bed, Jet-Driven Bed Reactor

120.0 g (0.52 mol) of dried crude3,4-ethylenedioxythiophene-2,5-dicarboxylic acid were placed in a glassreactor (diameter: 50 mm, total height: 730 mm, conical gas inlet with a10 mm gas distributor frit) and heated to a maximum reaction temperatureof 320° C. (heating rate: 2° C./min) in a stream of nitrogen (normalconditions). The starting material was reacted over a time of 100minutes. 32.0 g (0.139 mol) of the crude starting material had reacted.16.6 g of product were condensed out in a downstream cold trap. The3,4-ethyleneidoxythiophene content was >94% by weight.

Example 3 “Particle Powder” Fluidized Bed

3000 g of silica sand having a diameter of 160-250 mm were placed in aglass reactor (diameter: 95 mm, total height: 700 mm, gas distributorfrit: 95 mm) with an additional gas distributor and heated to a reactiontemperature of 280° C. in the silica bed with fluidization by means ofnitrogen. A mixture of 0.1351 mol of3,4-ethylenedioxythiophene-2,5-dicarboxylic acid and basic coppercarbonate in a mass ratio of 1:1 was introduced into thenitrogen-fluidized reactor over a time of 58 minutes. Thedecarboxylation proceeded there and the product was condensed in aplurality of cold traps connected in series. The yield was Y=83 mol % ata product purity determined by HPLC of more than 94% by weight. Thesolid discharged from the reactor and separated off in a cyclone can bereused as starting material.

1. A process for the thermal decarboxylation of3,4-ethylenedioxythiophene-2,5-dicarboxylic acid as starting material,comprising: reacting the starting material as a solid in the presence ofa plurality of fluidized-bed bodies, and wherein the reaction is carriedout in the absence of solvents, and discharging the decarboxylationproduct formed in the reaction from the reaction zone in gaseous form.2. The process as claimed in claim 1, wherein the decarboxylation iscarried out at a temperature of from 100 to 600° C.
 3. The processaccording to claim 1, wherein the process is carried out continuously ina bubble-forming, turbulent, jet-permeated fluidized bed or in aninternally or externally circulating fluidized bed.
 4. The process asclaimed in claim 1 wherein the reaction is carried out in the presenceof an inert auxiliary gas selected from the group consisting of noblegases, nitrogen, water vapor, carbon monoxide, carbon dioxide andmixtures thereof.
 5. The process as claimed in claim 1 wherein thereaction is carried out in a fluidized-bed reactor in which fluidizedbed bodies having a mean diameter (number average) greater than theparticle diameter of the dicarboxylic acid.
 6. The process according toclaim 5, wherein the fluidized bed bodies have a solids density ρ_(s) of0.5 g·cm⁻⁵<ρ_(s)<6 g·cm⁻³.
 7. The process according to claim 1 whereinthe fluidized bed bodies are used as heat transfer media wherein thefluidized bed bodies are preheated outside the reaction zone andcirculated through the reaction zone and comprise a catalytically activematerial.
 8. The process according to claim 7, wherein the catalyticallyactive material of the fluidized bed bodies comprises copper or a coppersalt.
 9. The process according to claims 1 wherein any solid carried outfrom the reaction zone by the gas stream is separated off from theproduct by means of a cyclone and/or filter.
 10. The process accordingto claims 1 wherein the unreacted solid starting material separated offfrom the product gas steam is recirculated batchwise or continuously tothe reaction zone.
 11. The process according to claim 8 wherein thecatalytically active material of the comprised CuCO₃.